How to Make a 12v Battery Charger with Auto Cut-Off

An LED chaser is an electronic circuit that lights LEDs one after another in a sequence. It creates a running or chasing light effect commonly seen in decorations and indicators.

Components Required:

• BC507 Transistor
• 12v Relay
• 1k ohm
• 15k ohm
• 1N4007 Diode
• 5408 Diode
• LED – green & red
• Potentiometer 10k (103)
• Connector
• 14V Power supply

Circuit Diagram:

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BC507 Transistor

The BC547 is a general-purpose NPN transistor made from silicon. It is mainly used where small signals need to be amplified or controlled. Because of its low cost, reliability, and availability, it is one of the most popular transistors in the world.

Basic Information

Parameter	Value
Type	NPN BJT
Package	TO-92
Material	Silicon
Operation	Amplifier / Switch
Polarity	NPN

Pin Configuration (TO-92)

When the flat side faces you and the legs point downward:

Base (B) – Control terminal

• Emitter (E) – Current exits

⚠️ Pin order may vary slightly by manufacturer, so always check the datasheet.

⚙️ Working Principle

The BC547 works on the principle of current amplification.

🔹 Basic Rule

A small current at the Base controls a large current between Collector and Emitter.

🟢 Switch Mode

• Base current = OFF → No C-E current
• Base current = ON → C-E current flows

Used for:

• LED ON/OFF control
• Relay driving (small relays)
• Digital switching

🔵 Amplifier Mode

• Weak AC signal applied at base
• Strong amplified signal obtained at collector

Used in:

• Audio amplifiers
• Microphone circuits
• Sensor signal amplification

📊 Electrical Characteristics

Parameter	Typical Value
Vceo (Max Collector-Emitter Voltage)	45 V
Ic max (Collector Current)	100 mA
hFE (Current Gain)	110 – 800
Power Dissipation	500 mW
Transition Frequency (ft)	~300 MHz

🔈 Gain Classification

BC547 comes in three gain versions:

Type	Gain (hFE)
BC547A	110 – 220
BC547B	200 – 450
BC547C	420 – 800

✔ Higher gain = better amplification

🧪 Applications of BC547

✔ Audio pre-amplifiers
✔ Signal amplifiers
✔ Microcontroller interfacing
✔ Relay driver circuits
✔ Light & temperature sensors
✔ FM transmitter circuits
✔ Logic switching circuits

🔄 BC547 vs Similar Transistors

Transistor	Type	Notes
BC547	NPN	Low noise, general purpose
BC548	NPN	Lower noise than BC547
BC549	NPN	Ultra-low noise
2N3904	NPN	American equivalent
C1815	NPN	Audio applications

⚠️ Important Notes

• Do not exceed 100 mA current
• Use base resistor to protect transistor
• Suitable only for low-power circuits
• Not recommended for high-current loads (motors)

How relay works

A relay is an electrically operated switch that uses an electromagnet to control a separate electrical circuit. When a small current flows through the relay’s coil, it creates a magnetic field that pulls a metal armature, causing the relay contacts to change their position. This action can close (NO – Normally Open) or open (NC – Normally Closed) a circuit, allowing a low-power signal to safely control a high-voltage or high-current load. When the coil current is switched off, the magnetic field collapses and a spring returns the contacts to their original state. Relays provide electrical isolation, protect control circuits, and are widely used in automation, motor control, and electronic systems.

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